The present invention relates to pavement markers of the cube corner reflex reflector type which are cleaned by the action of vehicular traffic on the roadway contacting the reflector, and in particular to such pavement markers which are suitable for use in snow areas and are, therefore, constructed so as to protect the reflector from contact with snowplow blades.
Pavement markers have become widely accepted as permanent installations for providing visible signals which mark traffic lanes and control the flow of traffic on roadways in connection with, or in place of, conventional painted traffic lines. While a large number of such markers employ reflectors which reflect light emanating from oncoming vehicles to provide a visible signal to the operators of such oncoming vehicles, other markers have been proposed which utilize an independent light source, such as an electric lamp located within the marker, to provide a signal visible from oncoming vehicles. The term "signal means" is employed herein to denote any such marker employing a reflector, a lamp or another light source or any arrangement which provides the desired visible signal.
A snowplowable version of such a prior art pavement marker is disclosed in U.S. Pat. No. 3,790,293, issued to S. A. Heenan et al. on Feb. 5, 1974, and U.S. Pat. No. 3,809,487, issued to R. M. Flanagan on May 7, 1974, both of which patents are commonly assigned herewith. In the arrangements used in those patents, a base member of relatively high-strength material, such as metal, includes a pair of laterally spaced apart keels which are permanently affixed to the roadway surface by insertion into grooves cut in the pavement, and a reflector body of synthetic resin material is affixed to the base member for selective removal and replacement without destruction of the base member. The base member is provided with inclined ramps for protecting the reflector body from encounters with snowplow blades.
In these prior art snowplowable pavement markers, an attempt was made to minimize the height of the pavement marker above the roadway surface by minimizing the height of the reflector body carried by the base, thereby to minimize the impact forces imparted to vehicle tires as they passed over the pavement marker. Indeed, in these prior art snowplowable pavement markers the maximum height of the marker above the roadway surface had been reduced as far as possible with existing reflex reflector bodies and installation techniques, consistent with obtaining satisfactory visibility of the pavement marker, but could not be reduced below about 0.72 inches above the pavement.
It has also been recognized in connection with these prior art pavement markers that the angle between the roadway surface and the inclined ramps of the base member should be minimized to minimize the impact forces imparted to the pavement marker and to the surrounding pavement by impact of snowplow blades with the inclined ramps of the pavement marker. While theoretically the ramp angle could be reduced as low as desired, the lower the angle the longer the ramp would have to be to maintain the same maximum height and, accordingly, the longer the keel members and the longer the grooves or recesses that would have to be cut in the pavement. The longer the grooves, the greater the weakening of the pavement and the greater the time and expense required to form the grooves. Furthermore, the longer the base member, the heavier and more expensive it is. Thus, these factors serve practically to limit the ramp angle that could be obtained with these prior art pavement markers to no lower than 6 degrees.
In addition, the prior art pavement markers were monodirectional devices. While bidirectional reflector bodies were available, in order to mount them in a metal base member for protection from impact with snowplow blades, it would be necessary to have inclined ramps extending from the reflector body in both directions. Thus, if the same ramp angle and maximum height above the roadway surface were to be maintained, it would be necessary virtually to double the length of the base member, with the attendant disadvantages discussed above.
It has been suggested in the prior art partially to recess the reflector elements below the level of the roadway surface as, for example, in U.S. Pat. No. 2,260,498, issued to L. M. Wise on Oct. 28, 1941, and U.S. Pat. No. 1,952,942, issued to D. E. Ross on Mar. 27, 1934. In Wise and Ross, because of the nature of the glass lens used, the portion of the lens disposed below the roadway would be wasted and inoperative. Further, the Wise pavement marker is a generally cylindrical body embedded in the pavement, with a part-conical inclined upper surface extending above the level of the roadway surface at an angle in excess of 25 degrees with the roadway surface, at which angle the impact forces of snowplow blades against the pavement marker are so great that they destroy the pavement marker and/or severely damage the surrounding pavement and snowplow blade. Reduction of the angle by increasing the diameter of the Wise pavement marker would require unacceptable increases in the size of the pavement recess and the weight of the pavement marker itself. Furthermore, the cross sectional outline of the Wise pavement marker body is not conducive to ready insertion into a recess which is cut or drilled in a finished pavement.
The Ross device suffers from basically the same disadvantages. Also these devices, as well as those of U.S. Pat. Nos. 3,836,275 (Finch) and 2,126,224 (Shaffer et al.), essentially try to minimize height by using small part-spherical bead-type reflectors. All have proven impractical and inefficient as a result of dirt build-up, since they are unaccessible to the wiping action of passing vehicle tires.